This invention relates to coreless induction furnaces and, in particular, relates to a system for simultaneously melting metal and holding molten metal for casting operations and the like which uses a plurality of coreless induction furnaces connected in series with a single AC power supply.
Coreless induction furnaces are, of course, wellknown. The coreless induction furnace is a "batch" type metal melting furnace, which simply means that it is regularly charged with finite quantities of cold melting stock consistent with its rated capacity. When the charge has been melted and superheated to the desired pouring temperature, the furnace has completed its melting duties and the molten metal is available for use. At this point, the power to the furnace is either turned off or reduced in order to maintain the temperature of the molten metal during pouring. When the desired amount of molten metal has been removed from the furnace, the next charge is put in and full power is applied to the furnace to begin the next melt cycle. Thus, a full melting cycle consists of the time to melt and superheat the charge plus the subsequent period of "lost" time for such things as removing slag, checking the temperature and chemical analysis of the melt (and adjusting it, if necessary), tapping the furnace, and putting in the next charge.
The production capability of such a furnace is more or less directly related to the ratio of melting time to the overall cycle time. If the "lost" time can be kept to a minimum, the effective utilization of the equipment will be high and the actual production rate obtained will approach the designed melting rate of the furnace. Utilization figures of 75% to 80% are considered good in the industry, and there are even instances where 90% to 95% utilization has been obtained.
There are, however, many melting operations where the amount of lost time per melt cycle is unavoidably high. The resulting utilization can be as low as 40% to 50%, with correspondingly low production rates. This may be due to a number of reasons, including the following:
(a) There may be a long metallurgical treatment period required before the metal can be used; PA1 (b) The particular molding system being used may require a large number of small ladles of molten metal; PA1 (c) There may be limitations in the molten metal handling system which lengthens the time to empty the furnace; and PA1 (d) The skill or working methods of the operating personnel.
Where these conditions occur, the use of a standard batch melt/batch pour coreless induction furnace becomes less convenient and less economical. Thus, it is an object of the invention to improve the operating characteristics of the coreless induction furnace in situations where the percentage of lost time is high and equipment utilization is low.
Several prior attempts have been made to reduce the percentage of lost time and increase furnace utilization. In one prior method, a holding furnace with its own power supply (usually of a lower power rating than the melting furnace) is provided, completely separate from the melting furnace. When the melt cycle is finished, molten metal is transferred quickly and usually in large quantities from the melting furnace to the holding furnace, and the melting furnace is re-charged and the next melting cycle begun. In this manner, the utilization of the melting furnace can be very high. Metallurgical treatment may be done in the holding furnace, and molten metal is stored at the desired chemical analysis and pouring temperature, available to supply the casting line at any convenient rate and quantity.
A second prior method, known as a "butterfly" system, uses two coreless induction furnaces connected to a single power supply through power transfer switches. When the melt cycle in one of the furnaces is complete, power is switched in its entirety to the other furnace and its melting cycle commences, while the first furnace is being tapped. However, with no power applied to the first furnace, the molten metal temperature will gradually decrease, so that it may be necessary to switch power back to the first furnace periodically to re-heat the molten metal and keep the pouring temperature differential within acceptable limits.
A third prior system utilizes two furnaces and two power supplies, with power transfer switches so that each of the furnaces can be connected to either power unit. One of the power supplies has a high power rating for melting, and the other a low power rating suitable for holding molten metal at the pour temperature. When the melt cycle in the first furnace is complete, the melting power supply is switched to the other furnace to commence its melt cycle, and the holding power supply is switched to the first furnace to maintain the temperature of the molten metal. The furnaces are alternated from melting to holding throughout the working day.
All of the prior methods utilize two separate furnaces with varying methods of proportioning power to the furnace being tapped while the other furnace is in the melting cycle. All of the prior methods improve furnace utilization with a higher production rate than is possible with a single furnace and single power unit, and also provide a higher degree in pouring flexibility to the user.
However, all of the prior methods have certain disadvantages. The first method requires a great deal of floor space, has a high initial cost, and requires metal to be transferred from a melting furnace to a holding furnace. The "buttlerfly" system causes wide variations in pouring temperatures. The third prior method involves high initial costs.
It is an object of the present invention to provide the same separation of function between melting furnace and holding furnace to increase furnace utilization and productivity, but without the disadvantages of prior techniques.